Stars as Laboratories for Fundamental Physics - MPP Theory Group

Nonstandard Neutrinos 255
kinematical impact of a mass on certain reactions such as nuclear decays
of the form (A, Z) → (A, Z + 1) e − ν e where the continuous energy
spectrum of the electrons originally revealed the emission of another
particle that carried away the remainder of the available energy. The
minimum amount of energy taken by the neutrino is the equivalent of
its mass so that the upper endpoint of the electron spectrum is a sensitive
measure for m νe . Actually, the most sensitive probe is the shape
of the electron spectrum just below its endpoint, not the value of the
endpoint itself. The best constraints are based on the tritium decay
3 H → 3 He + e − ν e with a maximum amount of kinetic energy for the
electron of Q = 18.6 keV. This unusually small Q-value ensures that a
large fraction of the electron counts appear near the endpoint (Boehm
and Vogel 1987; Winter 1991).
In Tab. 7.1 the results from several recent experiments are summarized
which had been motivated by the Moscow claim of 17 eV <
m νe < 40 eV (Boris et al. 1987). This range is clearly incompatible
with the more recent data which, however, find negative mass-squares.
This means that the endpoint spectra tend to be slightly deformed
in the opposite direction from what a neutrino mass would do. This
effect is particularly striking and significant for the Livermore experiment
where it is nearly impossible to blame it on a statistical fluctuation.
Therefore, at the present time one cannot escape the conclusion
that this experimental technique suffers from some unrecognized
systematic effect. This problem must be resolved before it will
become possible to extract a reliable bound on m νe although it appears
unlikely that an m νe in excess of 10 eV could be hidden by what-
Table 7.1. Summary of tritium β decay experiments.
Experiment m 2 ν e
± σ stat ± σ syst Reference
[eV 2 ]
Los Alamos −147 ± 68 ± 41 Robertson et al. (1991)
Tokyo −65 ± 85 ± 65 Kawakami et al. (1991)
Zürich −24 ± 48 ± 61 Holzschuh et al. (1992)
Mainz −39 ± 34 ± 15 Weinheimer et al. (1993)
Livermore −130 ± 20 ± 15 Stoeffl and Decman (1994)
Troitsk −18 ± 6 Belesev et al. (1994) a
a See Otten (1995) for a published description.